Most fireworks or pyrotechnics contain black powder to launch projectiles or pyrotechnic effects from reusable launch tubes or one-use disposable cardboard tubes. An aerial shell is an example of a fireworks projectile, where the shell, made of plastic or cardboard, is loaded with fireworks effects such as colored stars, hummers, whistles, etc. A comet is an example of an effect that is propelled from a launch tube. Projectiles and effects come in many different shapes and sizes, but all are commonly manufactured with a propellant charge as a component. The propellant charge, or “lift charge,” consists of a sieved fraction of granulated powder, for example, black powder.
Herein projectile is defined as shells or pyrotechnic effects which may be propelled by propellant powder, such as black powder.
The assembly or packaging of projectile lift charges includes propellant powder, such as black powder, of a specific sieve cut or granular size that is bagged and attached to the bottom of the projectile by string or tape. The bag, often made of paper or plastic film, must have within it an ignition source fired by external means. This ignition source may be an electric match with external wire leads, a fuse, or quick match. Quick match is a thin paper tube containing fine black powder that is specifically used for igniting devices with very short delays. These methods provide the means of igniting the lift charge and thereby launching the projectile.
Besides propelling projectiles, black powder may have a dual function of igniting time-fuses and/or the pyrotechnic effect itself. Shells usually contain a time-fuse, which serves as a time-delay element for ignition of the propellant charge within the shell. Effects may be coated with an easily ignitable outer layer or prime that takes fire readily from the burning black powder. Comets or stars are usually coated with prime and are ignited within the launch tube by the burning black powder.
Black powder, also referred to as gunpowder, includes an intimate mixture of potassium nitrate (KNO3), charcoal and sulfur, produced by a manufacturing technique called the Corning process. This process typically involves the grinding of the chemical components moistened with water, pressing the resulting mix into cakes that are dried, crushed and sieved.
Black powder typically burns rapidly at relatively low pressures when ignited, producing the necessary hot, and burning gases to propel the projectiles. This attribute is unique to black powder and black powder propellant is preferable for launching projectiles.
Burn rate is defined herein as the rate of conversion of propellent to combustion gases, or more specifically, the mass of propellant converted to combustion gases per second per unit area of burning surface.
Those knowledgeable in the art of pyrotechnics understand that fine black powder can burn more quickly than coarse granular black powder, simply because fine powder has a greater available surface area for burning.
Projectiles are typically launched from tubes or mortars at pressures between 50 and 150 psi, but more desirably between 80 and 120 psi. These pressures are considered the operating pressures of black powder in launching projectiles. If the pressure is too low, the projectile may not be sufficiently accelerated and the apex height will not be reached. If the pressure is too high, the projectile may be destroyed or damaged, or worse, the mortar may rupture. The above operating pressures may be suitable for achieving reasonable apex heights without risk of damage to the projectile or mortar.
The efficiency of the black powder to perform as a lift agent may greatly depend on its confinement. For example, black powder when laid out in open air may burn very slowly, perhaps taking several seconds to burn. When confined, black powder will typically combust at a rate resembling an explosion, completely burning within milliseconds. The greater the amount of free space volume (less confinement), or ullage, within the lift charge, the slower the black powder reaches a desired operating pressure. Likewise, if the confinement is too great, or the free space volume is too small, the actual black powder burn pressures will rapidly go beyond the desired operating pressure, which could cause rupture of the launch tube or damage to the projectile.
This effect of confinement is ascribed to what is referred to as pressure effects on burn rate. Burning black powder produces hot gases, which when confined in a chamber or vessel, will result in an increase of gas pressure. The black powder in turn will burn faster, thus producing still greater pressures. This positive feedback process is typically referred to as the pressure effect on burn rate. The burning of black powder is a complex, multi-step chemical and combustion process that occurs at the solid-gas interface. Higher pressures compress the gas phase at the interface thereby increasing the heat and mass transport of reaction species.
Other propellants, such as smokeless powders, require great confinement and pressures to achieve relatively fast burn rates. Smokeless powders are nitrocellulose-based formulations suitable for propelling bullets and heavy projectiles at high velocities. For example, exemplary nitrocellulose-based smokeless powders are described in U.S. Pat. No. 701,591 which is hereby incorporated by reference. The preferable operating pressures of such propellants are in the thousands of psi, which far exceeds that of conventional fireworks of about 50 to 150 psi.
Propellants other than smokeless powder also experience pressure effects. Examples are solid rocket propellants and liquid propellant systems. These examples rely on the chemical conversion of solid or liquid propellants to hot gases via a combustion process, which accelerates with increasing pressures (i.e. pressure effect). The operating pressures of these examples, however, are greatly unsuitable for lifting pyrotechnic projectiles. Solid rocket propellants may require hundreds or thousands of psi to operate effectively. Similarly, liquid motors operate at relatively high pressures and indeed are inherently unsuitable and impractical as pyrotechnic lift propellants. Black powder is uniquely capable of producing the lifting at an operating pressure range suitable for pyrotechnics.
One problem with black powder as a lift propellant is that it produces copious amounts of smoke. It is well known that more than half of the combustion products are solids (e.g. potassium oxides, potassium carbonates) as well as noxious sulfur oxides from the combustion of sulfur. Because such combustion happens near the ground when lifting projectiles, the smoke produced by a fireworks display impacts the air quality at the ground level. The audience and crew can be affected by such smoke.
Attempts to use low smoke alternatives to black powder have proved difficult or impractical. U.S. Pat. No. 7,104,199 provides the expertise to utilize high pressures for fast combustion of nitrocellulose-based smokeless powders to launch pyrotechnic projectiles, but acknowledges that burning propellants lacks the heat to ignite delay fuses and primes of the projectiles. U.S. Pat. No. 8,402,893 in a similar fashion, uses a high pressure confinement technique to burn smokeless powder at its proper high operating pressures, and bleeds the resulting gases via small ports to step the pressure down to lower a lower operating pressures suitable for propelling firework projectiles.
The four basic disadvantages of this prior art method includes: (1) the high pressures required to burn smokeless powders requires a preferred metal vessel strong enough to handle the high pressures. This is expensive when compared to traditional systems employing cheap paper, fiberboard, cardboard or plastic; (2) the use of metal in the primary containment may be potentially unsafe for the user or audience; and (3) the step-down depressurization of burning gases from high pressure vessel to the lifting chamber (the mortar) via small port holes would undesirably cool the gases. This cooling effect is universally known in gas dynamics as the Joule-Thomson Effect, and would impede the ignition of time delay fuses and primes of the pyrotechnic effect; (4) the separation of the lift agent from the pyrotechnic projectile requires the development of a “lift kit,” which will result in an increased in time of preparing displays, increasing maintenance of the mortar tubes, and thus increasing labor and capital costs.
A non-propellant system using pressurized air together with electronic timed-delay fuses to launch projectiles is detailed in U.S. Pat. Nos. 5,627,338, 5,339,741, and 5,282,455. The inherent disadvantages of these prior art systems include high capital, maintenance and operating costs. In addition, special methods are required to ignite time delay fuses and primes of the projectiles since the propelling gases are cool.
Another method described in U.S. Pat. No. 6,645,325 employs an organic catalyst to increase the burn rate of raw nitrocellulose fibers. The burn rate increase is such that the nitrocellulose can burn at typical firework operating pressures. The organic catalyst, however, proved to be expensive to manufacture and unstable to moist air, making it impractical for use in the fireworks industry.
Therefore there continues to be a need to provide new methods and pyrotechnic devices for improving the efficiency of lift charges including propellant powder, such as black powder as the propelling agent in launching of firework projectiles while avoiding adverse affects such as adversely affecting the ignition of fuses and prime associated with the firework projectiles or pyrotechnic effects.
It is therefore an object of the invention to provide a pyrotechnic device and a method for making the same including an improved lift charge package including propellant powder, such as black powder as the propelling agent, to launch firework projectiles while avoiding adversely affecting the ignition of fuses and prime associated with the firework projectiles or pyrotechnic effects.
Another object of this invention is to increase the efficiency of the black powder as the propelling agent in the launching of firework projectiles. It has been unexpectedly found that in some embodiments the amount of needed amount of black powder compared to the amount used in traditional methods may be reduced by up to about 81%, and where such reduction may be accomplished a reliable manner without compromise on the ignition of time fuses and prime.
These and other objects, aspects and features of the invention will be better understood from a detailed description of the preferred embodiments of the invention which are further described below in conjunction with the accompanying Figures.